Making a penetration in a sheet metal wall is a common requirement. Such a penetration is used for example to enable coupling of one end of a conduit to the wall of a fluid storage tank. This enables connection with appliances or other equipment that use or consume fluid held within the storage tank.
Many methods used to perform the task of creating a sealed joint at the penetration are prone to corrosion or introduce significant stresses in the materials.
A common method of forming the joint is by welding in which work pieces are melted and filler material is added to form a weld pool. It has been found, for example for stainless steel articles, that one of the causes for corrosion at a welded joint is the depletion of chromium from the stainless steel as a result of the heat addition. Pickling and/or passivating the surface of the welded joint to remove heat tint and high temperature scale from the weld site may overcome this problem.
The use of manual welding techniques to join a stainless steel tube to a sheet runs into difficulties when the material thicknesses become small, say less than 3 mm, for example 0.55 mm, and great skill is required to achieve a uniform and reliable welded joint. Moreover, the high heat addition for both automated and manual welding usually requires significant post weld treatment.
An example of a method for creating a boss in a sheet article is provided by U.S. Pat. No. 3,939,683, where the frictional heating of a rotating tool is used to pierce the wall of a sheet and simultaneously create a bossed circular hole from displaced softened metal capable of receiving a thread (e.g. see http://www.thermdrill.com). This method requires sufficient heat input and volume of material to be softened to form the bossed hole. The method provides a solid and separable joint, but is understood to require sheet wall thickness of approximately 1 mm or more and involves significant heat addition. Further this method does not form a join between two initially separate articles.
The above described problems in the background art are not intended to limit the application of the method as disclosed herein.